Documents EP 0 665 079 A1 and EP 2 215 329 A1 (resp. WO 2009/065739 A1) describe the principles for welding rotors for power generation (gas turbines, steam turbines, generators), which comprise a plurality of rotor discs arranged along a rotor axis, thereby using tungsten inert gas (TIG) welding, especially very narrow gap TIG for the root area of the welding joint, and afterwards submerged arc welding (SAW) to fill the weld seam.
FIG. 1 (similar to EP 0 665 079 A1) shows a prior art welding joint with a welding seam 13 between two adjacent rotor discs 11 and 12 of a welded rotor 10. Before welding the two discs 11 and 12 abut with a centering step joint 20 of height h, thereby creating an extreme narrow gap 19, which is defined by extreme narrow gap component faces 23 and 24 of the discs 11 and 12, and a narrow gap 14, which is defined by narrow gap component faces 15 and 16 of the discs 11 and 12.
The extreme narrow gap 19 is filled with TIG welding seams 25, each welding seam extending over the full extreme narrow gap width b. The narrow gap 14 is filled with SAW welding seams 17, 18, which are smaller than the narrow gap width a and are alternating butt against the opposing narrow gap component faces 15 and 16, and overlap in the middle of the narrow gap 14. During TIG welding a fusing area 22 at the root of the welding joint is fused. Furthermore, a relieving ridge 21 with a relieving ridge width c and height d and relieving ridge angle a is provided at the root of the welding joint.
The extreme narrow gap TIG weld (welding seams 25) are welded typically in a vertical stack of rotor discs (see for example FIG. 2 of WO 2009/065739 A1) and the subsequently filled SAW weld (SAW welding seams 17, 18) is filled in horizontal position (see for example FIG. 3 of WO 2009/065739 A1). The TIG-SAW transition is shown with a U type weld joint preparation for the SAW area.
The Typical Dimensions of Such a Prior Art Welding Joint Are:
Typical width of TIG extreme narrow gap: b=10 mm. Typical width of SAW narrow gap: a=17 mm.
Thus, according to the prior art, in a first step, the forged and NDT tested discs are machined for weld seam preparation.
Afterwards the discs are stapled on top of each other and their run-out relative to each other and as a whole is checked and, if necessary, adjusted.
Now, with the discs in vertical position the root of the weld is melted without weld filler using TIG welding.
After this the weld height is increased to allow tilting of the rotor in horizontal position using very narrow gap TIG welding with parent metal weld filler.
After tilting the rotor in horizontal position, the welding is finalized, filling the weld using SAW welding.
Finally the welds of the rotor are NDT checked using ultrasonic (US) testing.
As mentioned above already, the welding of rotors for power generation equipment is described in more details in document s EP 0 665 079 A1 and EP 2 215 329 A1.
The welding seam geometries recommended for SAW welding are given in DIN EN ISO 9692-2 tables 1 and 2. The narrow gap from TIG needs to be filled practically to the bottom of the U of the SAW joint preparation. This is not always achievable due to oxygen pick-up without additional machining, or a solution for the transition has to be found, which allows some scatter for the bottom of the U shape joint preparation.
For proper NDT results of the transition area TIG to SAW the inert gas flow would need to be increased by a factor of 3 to 5, if the narrow gap is broadened up towards the end of the narrow gap for TIG.
With the U joint preparation for SAW as well as using U with V root preparation (1.3.7 in DIN EN ISO 9692-2) occasionally NDT indications above acceptance limit are found, which need costly rework.
Thus, the prior art technology for the transition zone between TIG and SAW would need machining after TIG welding and before SAW welding to assure best transition area non-destructive testing (NDT) results. In this case for SAW a weld joint preparation according DIN EN ISO 9692-2 would be done after TIG welding is completed, with characteristic length parameters i, k, l, m, Radius r, and angle β (see FIG. 2).
However, this additional machining after TIG welding is time consuming and increases manufacturing costs.